1. Field of the Invention
This invention relates to a method of electromagnetic stirring in continuous metal casting processes, and more particularly to a method of electromagnetic stirring in which alternating currents of different frequencies are applied to a set of exciting coils thereby to induce electromagnetic stirring actions which can effectively stir molten steel in unsolidified portions of a continuously cast strand to reduce center segregation for manufacturing cast products of good quality.
2. Description of the Prior Art
There have already been proposed electromagnetic stirring methods of this sort, for example, in Japanese Pat. Publication No. 52-4495, wherein the unsolidified portions of molten metal of a continuously cast strand (hereinafter referred to as "c. c. strand" for brevity) are electromagnetically stirred by a magnetic field induced by alternating current which is intermittently applied to an exciting coil. This method is intended to produce a regular flow of molten metal in the time period when alternating current flows through the exciting coil, and to produce temporary inertial turbulence in the regular flow of the molten metal by interruption of the alternating current, thus utilizing the mixing and stirring actions of the rectified and turbulent flows. One problem with this method is that, in the period of regular flow which exists invariably by intermittent application of alternating current, a distinct white band appears due to the rotational flows which take place during the regular flow period, resulting in the acceleration of dense segregation in the core portion of the molten metal. Also proposed in Japanese Pat. Publication No. 53-6932 is a stirring method using an electromagnetic stirrer for applying electromagnetic force to the unsolidified portion at the center of continuously cast steel, switching the direction of current to be applied to the electromagnetic stirrer. This method, however, also has a drawback in that, when current in one direction is initially applied to the molten steel for an extended period of time, a distinct white band appears due to the regular flow, and when current is applied to the molten steel for a short period of time, molten steel flow is obstructed by an abrupt change in stirring direction. Therefore it is difficult to make the temperature of the molten pool uniform, thus hindering the production of an equiaxed crystal zone.